X-ray intensifying screens are generally used in conjunction with silver halide photographic films and serve to enhance the image formed on that film. Phosphors, which are the active component of X-ray screens, are legion in number and include the tungstates, the oxysulfides, and the oxybromides, among others.
Particularly efficient phosphors which may be used in the preparation of an X-ray intensifying screen are the tantalates described by Brixner in U.S. Pat. No. 4,225,623. These phosphors are based on yttrium, lutetium, and gadolinium tantalates of the M' monoclinic form and are generally activated with rare earths such as terbium, thulium and niobium, for example, as well described in the aforementioned reference. Since these phosphors have a high X-ray stopping power, they are presently widely used for the preparation of these intensifying screens and the method for their preparation includes the mixing of ingredients followed by firing this mixture to form the phosphor crystal lattice itself.
In the process of preparing tantalate phosphors, it has been found that alkali halides and combinations of alkali halides and alkaline earth halides are suitable for use as fluxes in increasing the reaction rate between oxides of yttrium and tantalum. A problem associated with the use of halides is their low melting points and the reactivity of these compounds with materials of construction used in furnaces and crucibles which contain the reaction mixture. The reaction between yttrium oxide and oxides of tantalum and niobium is best run at temperatures above 1200.degree. C. At these temperatures, lithium and sodium chlorides are within 250.degree. C. of their boiling points. Consequently, significant amounts of the chlorides are present as highly reactive gases in the furnace and the crucibles holding the reaction mixture. This condition can lead to shortened equipment lifetimes. Alkali sulfates are more stable in this regard but give phosphor with lower efficiency due to the effect of flux decomposition products. Lithium and sodium sulfates decompose, at the high reaction temperature used to obtain the phosphor, to lithium and sodium oxide, respectively, and sulfur trioxide. To minimize the effect of the decomposition products, small amounts of a sequesterant such as alkali metal metasilicate, e.g., lithium metasilicate, etc., is added to react with the alkali oxide and produce lithium orthosilicate.
It has been found that tantalate phosphors with very high efficiency can be produced while minimizing effects of flux and decomposition products by combining the stable sulfates with the reactive halides in the presence of small amounts of a sequesterant compound.